Stoker



1945- s. w. GUTHRIE 2,366,783

STOKER I Filed Nov. 5, 1943 INVENT OR. Jpgze ((9! Goff/21E:

v Patent ed Jan. 9, 1 945 I s'roxna Speer W. Guthrie, Indiana, Pa., asslgnor to Pennsylvania Bituminous Coal Research, Inc.,

Altoona, Pa., a corporation of Pennsylvania Application November 1943, Serial No. 509,071

' 3 Claims. (am no-45) This invention relates t'o'stokers, more particularly to underi'eed stokers which operate on the principle of partial oxidation of coal prior to its undergoing active combustion.

Underfeed stokers as ordinarily constructed do not operate satisfactorily with strongly coking coals, such as the Pennsylvania. coking coals.

That is, when it is attempted to burn a strongly coking coal in an underfeed stokerof customary construction there is a strong tendency for unburned coke to be formed and to accumulate and build up above the retort in a generally columnar form known as a coke tree. Coke trees burn primarily at the .base. and the presence of a coke tree results in unsatisfactory combustion, espe-- cially when the demand on the heating unit is not great, they slow down the response of the furnace to demand for heat' and cause overshooting of thermostatic control, and they make it difficult to maintain low hold-fire rates without danger of the fire goin out. In fact, coke --tree formation may become so extensive as ac-. tually to extinguish the fire or to push open the furnace door. a

A method of and apparatus for avoiding the formation of coke trees from coking coals by mildly oxidizing the coal before active combustion begins is described and claimed in an appligation must be avoided, i. e., it is necessary to maintain substantial uniformity of coal size distribution across the section while mixing it intimately with the oxidizing agent. These features, result in subjecting all of the coal uniformly to the action of the preoxidizing air or other oxidizing agent, and observance of them is necessary because it has been found that coke tree suppression is not attained satisfactorily'if the coal is not thoroughly mixed with the-air 'or if the air distribution is not uniform due to channeling or segregation of the coal into regions containing largely fines and regions containing largely coarse particles. And as indicated above, the coal must be heated but not sufilciently to render it plastic,

and enough oxidizing agent must be supplied to producethe desired oxidation before coking of the coal be ins.

Although the necessary elevation and mixing of the coal without segregation in the preoxidation zone may be accomplished satisfactorily in various ways, the simplest and most effective means for doing so is a vertical screw. Likewise, al-

. though the oxidizing agent might be introduced into the coal at any point prior to the preoxidation zone, it is preferred to supply the oxidizing agent through a supplementary, or preoxidizing,

cation filed by Theodore S. Spicer and Calvert C. Wright on April 15, 1941, Serial No. 388,626.

As the coal moves upwardly into and through the lower end of the retort of the stoker it is heated by conduction of heat downwardly from the actively burning coal, and in that portion of the column of coal in which the coal is heated but is 'not yet plastic, 1. e., where coking has not begun, the coal is oxidized to an extent such as to reduce but not destroy its coking power. Thisoxidation is chiefly a surface phenomenon but it does not involve active combustion. This isthat the preoxidation method easily and satisfactorily represses coke tree formation provided" certain critical details are observed- In the first place, the coal must be mixed intimately with the oxidizing agent as it moves upwardly through the preoxidation zone. Second, 'as the coal is mixed and elevated anysubstantial segre,

tuyre situated below the regular, or active combustion; tuyeres at a distance'such that the agent enters the coal substantially at the lower end of the preoxidation zone.

It has been found in actual practice that in .the operation of such a stoker there may bea tendency for coal to enter the supplementary tuyre, even to the extent of closing the tuyre opening. This is serious because it decreases the amount of air supplied by the supplementary tuyere' to the coal and'coke trees result because the coal is not oxidized sufficiently. Moreover,"

continued operation may completely, fill the the time of this invention, however, no satisfactory solution of that difiiculty'had been found. The major object of this invention is to provide a stoker constructed and operated in accordance I with the preoxidation principle with simple, inexpensive and automatic means for keeping the supplementary, or preoxidation, tuyr free from coal.

Other'objects will appear from-the following description.

The invention will be described with reference to the accompanying drawing in which Fig. 1-

.by providing a horizontally movable scraper member which ismtated in the tuyre opening to sweep it free from coal that may have found its way into the opening. This may be accomplished, for example, by connecting a scraper member to the vertical screw so that when'the screw rotates during the on periods of the stoker the scraper continuously and automatically acts to keep the supplementary tuyre open.

Having reierence now to the drawing, there is shown a preoxidation type of stoker comprising a retort I of conventional form provided with one or more main combustion tuyres which form the active combustion zone in the manner customary in the art. In the embodiment shown there are two such tuyeres, shown at 2 and 3, Fig. l. The retort is surrounded by a-wind box 4 supplied with air or other oxidizing agent from a main airduct 5 in anysuitable way, as by a motor driven blower. Coal is introduced to the retort by a horizontal screw 6 operating in a conduit I. The blower which supplies air to main duct 5 and the'screw conveyor 6 are operated in any suitable manner; such elements form no part of this invention, and because they are well understood in the art they do not require illustration or further description.

Coal is delivered by screw conveyor 6 to the vided as shown in Fig. 1 for the usual purpose of preventingback pressure at this point.

The upper, or active combustion, tuyeres 2 and 3 are, as will be observed from Fig. 1 of the usual sectional construction which forms a series of spaced openings between webs 2a and 3a. In accordance with the present invention, however, the preoxidation tuyere I4 has a continuously unobstructed opening, as shown in Fig. 1, to permit a scraper member I! to enter the tuyere opening and to pass completely around it to sweep from the opening any pieces of coal or other material that may be forced into the tuyere opening by the action of the vertical screw when it'operates to lift the coal upwardly into the retort.

' The scrapervmay be mounted and actuated in various ways. In accordance with the preferred embodiment of the invention, however, scraper bottom of the retort where itis picked up and moved upwardly by a vertical screw conveyor 8 operated by a' gear 9 and a pinion Ill which may suitably be driven by the motor which operates screw 6. During the on periods of the stoker screw conveyor 8 moves the coal progressively and continuously upwardly through the 'retort intothe active combustion zone and in so doing the coal is mixed without segregation of particles or pieces of varying size.

Disposed in wind box 4 is a partition I I which divides it into an upper air chamber I2 through which air. is supplied to the active combustion tuyres 2 and 3, and a lower air chamber I3 through which air is supplied to the preoxidation tuyere I4. Chambers I2 and I3 act as conduits for supplying air to the main and supple- Mounted at the juncture of mentary tuyeres. wind box I and the main air ductfor pivotal movement about the end of partition II is a butterfly valve It. By moving it appropriately, as indicated by the broken line positions, Fig. 1, the air supplied through duct 5 may be apportioned in any desired manner between the upper and lower air chambers I2 and I3. This butterfly valve I5 may be adjusted in various ways, e. g., manually, according to need. Thus, all of the. air may be passed to the upper tuyeres 2 and 3, or all, of it may be passed to the preoxidation tuyere I4. In ordinary practice, however, valve I5 will be adjusted to supply an amount of air tothe preoxidation tuyerev I4 which efiects. preoxidation sufiicient to prevent coke tree formation, and air necessary for combustion is passed directlyto the active combustion zone through upper air chamber I2 and tuyeres 2 and 3. A smoke back opening I6 may be pro-- rectly, as by welding, to the blade of the screw.

at a point such that the scraper will move continuously through the tuyere opening under the action of the screw, as shown particularly in Fig. 1. In the embodiment shown in the drawing the scraper is in the form of a hook, or claw, and it is connected to the screw so that its concave face moves forwardly when the screw ror tates, as indicated by the arrow, Fig. 2.

The operation of a stoker constructed in accordance .with the drawing will be understood picked up by screw conveyor 8 and moved upwardly toward the active combustion zone. Air

for'preoxidation enters tuyere I4 which is situated at a level such that the temperature of the coal is suflicient for initiation of the desired preoxidation but insufilcient to make it plastic.

. Underthe combined action of mixing of conveyor 8 without substantial segregation and of the air introduced through tuyre It, the coal is sufllciently oxidized by the time it reaches the plastic zone, which lies at or immediately above the upper end of conveyor 8. that coke treesdo not form when the coal burns in the active combustion zone, 1. e., in the region of tuyeres 2 and 3. Meanwhile, rotation of screw 8 continuously moves scraper Il through the opening of tuyere I4 to keep it free from coal that may be pushed into it under the thrust of the screw. In this way the tuyere continues to function without interference to repress coke tree formation.

Various modifications are, of course, permissible. For example, the scraper member-may assume other forms or may be connected in other I ways than shown in the drawing."

According to the provisions of the patent statutes, I haveexplained the construction and practiced otherwise than as specifically illustrated and described. I

I claim:

1. An underfeed stoker comprising the combination of a substantially vertical retort provided with a main combustion tuyereat the upper portion where active combustion occurs, means for supplying coal to the lower end of said retort, a vertical screw disposed in said retort for moving coal upwardly in the retort while mixing it thoroughly and maintaining substantial uniformity of size distribution, the upper end of said screw being disposed below the plane of said tuyre, a supplementary tuye're having a continuously unobstructed opening disposed in the retort wall below said main combustion tuyre for supplying an oxidizing agent to the coal engaged by said screw in a region where the coal is heated but not plastic, conduits for supplying said oxidizing agent communicatingwith the said tuyres, and a scraper member connected to said screw and extending intosaid supplementary tuyre opening to rotate with the screw and sweep coal from said opening.

2. An underfeed stoker comprising the combination of a substantially vertical retort provided with a main combustion tuyere at the upper portion where active combustion occurs, means tuyre, a supplementary tuyere having a continuously unobstructed opening disposed in the retort wall below said main combustion tuyere for supplying an oxidizing agent to the coal engaged by said screw in a region where the coal is heated but not plastic, conduits for supplying said oxidizing agent to the said tuyres, and a scraper member connected to the blade of said screw and extending. into said supplementary tuyere opening to rotate with the screw and sweep coal from said opening.

3. An underfeed stoker comprisingthe combination of a substantially vertical retort provided tuyre, a supplementary tuyere having a conthe retort wall below said main combustion,

for supplying coal to the lower end of said retort,

a vertical screw disposed in said retortfor moving coal upwardly in the retort while mixing it thoroughly and maintaining substantial uniformity of size distribution, the upper end of said screw being disposed below the plane of said tinuously unobstructed opening disposed in tuyere for supplying air to the coal' engaged by said screw in' a region where the coal is heated but not plastic, a wind box communicating with said tuyeres for supplying air to them, and a scrapermember connected to the blade of said screw and extending into said sup- SPEER' W. GUTHRIE. 

